DESCRIPTION For a long time it has been thought that the dendrites of neurons are simple passive integrators of synaptic input. However, many recent in vitro studies have revealed that dendrites of neurons are not passive but instead have active conductances that confer non-linear properties. The physiological relevance of dendritic active conductances in the intact brain where the intrinsic patterns of network activity of neurons are preserved as an important topic that requires study in an in vivo preparation. This proposal will investigate the functional role of active conductances found in the dendrites of CA1 pyramidal cells in the hippocampus in vivo. The experiments combine the control and resolution of single cell intracellular recordings with multi-site extracellular measurements of both single cell and population activity. Intracellular recordings from the soma or dendrites of neurons permit the measurement of the sub-threshold variations in membrane potential that result from synaptic input and also permit direct control of the membrane potential. Extracellular recordings using multi-site electrode arrays allow the observation of both the spatial and temporal patterns of activity in both single cells and the hippocampal network as a observation of both the spatial and temporal patterns of activity in both single cells and the hippocampal network as a whole. The two main questions these experiments will address are: 1) Do physiological patterns of network activity initiate active conductance-dependent dendritic action potentials? 2) Do patterns of network activity facilitate modification in synaptic strength or neuronal excitability that depend on dendritic action potentials? The answers to these questions will help integrate data collected from in vitro preparations and provide clues about how the brain processes and stores of information during the learning and recall of memories.